Method of pulp production



United States Patent METHOD or PULP PRODUCTION John Francis Hooper and Ben Thoburn Briggs, Shelton, Wash., assignors to Rayonier Incorporated, Shelton, Wash., a corporation of Delaware No Drawing. Application March 12, 1952, Serial No. 276,224

8 Claims. (Cl. 9211) This invention relates to the production of acid sulfite precooked and alkaline digested pulps, and has for its object the provision of an improved method of producing such pulps. More particularly, it is the object of our invention to provide an efficient method of producing cellulose pulp of varying degrees of purity, including high-alpha cellulose, under such conditions that the heat values and the chemicals from both the acid and the alkaline stages are recoverable with greater efliciency.

In accordance with our invention, we subject wood chips to an acid sulfite precooking digestion with fortified acid sulfite liquor from a similar previous digestion, remove the major portion of the spent acid sulfite liquor from the chips, and subject the precooked unwashed chips to an alkaline purification of any suitable type. In an advantageous embodiment of our invention, we separate from the acid sulfite precooked chips all of the spent liquor that can be removed, as by draining, and reuse all or nearly all of the spent drained liquor in a subsequent acid sulfite precooking after fortification with the necessary amounts of base, sulfur dioxide, and addition of water, and subject the drained chips with adhering and imbibed acid sulfite liquor, together with such of the drained spent liquor as may be desired, to an alkaline digestion. This reuse of spent acid sulfite liquor is repeated indefinitely and the only liquor which we prefer to escape the acid system is that remaining with the drained chips, plus any minor amount of drained liquor Which may be added to the alkaline stage digestion charge. The drained but unwashed chips are then subjected to alkaline cooking, either in the same digestor or in a separate vessel.

It is, of course, well known to subject wood chips to a two-stage digestion, one consisting of an acidsulfite precooking and the other an alkaline digestion. In our invention, however, we recycle within the acid system all or nearly all of the drainable spent acid liquor used in the acid sulfite precooking, and depend essentially only on the imbibed and adhering undrained liquor left with the chips to remove from the acid system, once equilibrium is reached, such weight of solids as is dissolved during the acid precook, thus avoiding the necessity in present practices of disposing of this acid waste liquor directly to the sewer or to evaporators. We may remove a small proportion of the separated spent liquor and use it in the alkaline digestion. It is contemplated also that a small amount of the drained acid sulfite liquor may be remoyed from the acid circuit, preferably before fortification, and disposed of to waste or sent to the evaporators.

In a preferred practice of our invention, the chips are drained at the end of the precooking cycle and most, if not all, of the drained spent acid liquor is saved for reuse in a subsequent precooking operation after fortification with base, for example soda, S02, and the required amount of water to make the acid up to the initial volume. The balance of the drained spent acid, if any, is sent to the alkaline stage after stripping off free S02.

After "ice draining all possible acid liquor from the precooked chips, we may reduce acid vapors associated with the chips by evacuation or by steam flushing since residual acids neutralize alkaline materials to be used in the subsequent digestion.

The composition of the acid sulfite precook liquor is not critical. We commonly use soda-base acids ranging from more or less normal combined and free S02 content (0.8% combined, 6% free) to extremely low combined and high free (0.0% combined by test, 16% free) but have on occasion gone to much higher and lower combined and free without difficulty. Regardless of variation in the combined and free content, when equilibrium has once been established, all acid liquors are alike in containing sufficient organic solids so that the amount of spent liquor remaining with the drained partially cooked chips carries with it either exactly the amount of woody matter dissolved in the precooking stage or very nearly this amount.

The time and temperature of the acid sulfite pretreatment depend on the results desired and ordinarily fall within the range of one to five hours at to C.

The severity of the acid pretreatment determines the amount of material removed from the chips during this treatment. We normally regulate conditions to dissolve from 30 to 50% of the woody material, but occasionally remove as much as 60% or as little as 25% of the wood weight.

If the alkaline stage is to be carried out in the same digester as the acid precook, the alkaline liquor is pumped directly to the digester after removing as much as possible of the acid vapors. It is, however, possible, if desired, to transfer the unwashed precooked chips to a second vessel in which the alkaline digestion is made.

The alkaline cook or digestion may be made in liquors of widely difierent type, for example, such alkaline liquors as sodium hydroxide (soda cook), sodium hydroxidesodium sulfide (kraft cook), sodium hydroxide-sodium sulfide-sodium sulfite, sodium sulfide-sodium carbonate, sodium carbonate-sodium sulfite, sodium sulfite-sodium sulfide (with NaOH up to 15%) and, in general, any alkaline liquor which can, under proper conditions, be utilized to pulp wood is suitable for our purpose. However, since the chips are already partially delignified in the acidsulfite precook, it is possible to use milder conditions in the alkaline digestion than is usually possible in normal single-stage digestions.

In the ordinary single-stage alkaline cooking processes, it is customary to recycle a portion of the alkaline waste liquor (black liquor) back through the alkaline cook in order to save steam by building up total solids and to make use of the residual pulping potentiality of the waste liquor. Likewise in our process, it is possible and usually economically desirable to reuse some portion of the alkaline waste liquor in the alkaline stage digester charge.

Alkaline stage conditions must be adjusted depending on acid stage conditions. Normally the NazOzwood ratio falls within the range 0.10 to 0.30 but we have, on occasion, made cooks outside of this range. Normally, it is possible to use a low NazOzwood ratio when acid stage conditions have been such as to effect considerable delignification, and when the alkaline stage liquor is high in caustic or sulfide content. Conversely, if the amount of wood material dissolved in the acid stage is low, or if the alkaline liquor contains high proportions of sulfite or carbonate, it may be desirable to use a higher NazOzwood ratio.

Time and temperature conditions in the alkaline stage likewise are regulated depending on the precooking conditions used, it being generally true that longer times and higher temperatures are used when little delignification has occurred in the acid stage. Generally, the maximum temperatures fall within the range 130 to 180 C. and the time at maximum temperature within the range of 1 to 4 hours.

The following examples illustrate operations carried out in accordance with our invention with the use of soda base acid sulfite liquor and alkaline liquor:

Example 1 Hemlock chips were charged to the digester and sufficient acid sulfite liquor was charged to give a total liquidzwood ratio 1 of 5.3 l./kg. As charged to the digester the acid was characterized as follows: total solids 13.6%, combined S02 by test 0.05%, free S02 12.4%. The high total solids content was achieved by using essentially all of the drainable spent acid from an earlier similar precook on another. digester charge. The digester was steamed indirectly to 110 C. and after 2.0 hrs. at this temperature, the spent acid was drained to an accumulator for use in a subsequent precook and the digester pressure was reduced to atmospheric. The spent acid was found to contain 16.9% total solids and a small simultaneous basket cook indicated 65.6% precook yield. Sufiicient organics-free kraft liquor of 25% sulfidity was pumped into the digester to give 0.197 active alkali at 4.7 total liquidzwood ratio (based on weight of original wood). The digester was steamed indirectly to 160 C. and after 2.0 hrs. at 160 C. was blown. Characteristics of the pulp were as follows:

Alpha cellulose, 96.6% TAPPI Permanganate No. 7.0

In this example, where no black liquor solids were recycled through the alkaline stage and where the liquid: wood ratio in the alkaline stage was 4.7, the total solids in the efiiuent alkaline black liquor amounted to 16.3%.

Example 2 This example illustrates the application of our invention in a case where the alkaline stage was carried out in kraft green liquor (NaeS-NazCOs) but conditions otherwise were much like those in Example 1. Precook conditions were nominally like those in the preceding example except that the starting acid analyzed 0.00% combined S02, 12.7% free S02, and 11.4% total solids.

At the end of the precook, the spent acid containing and the alkaline black liquor contained 15.8% total solids.

Example 3 The following example illustrates the application of our invention to a cooking operation in which the acid and alkaline cooks are made in separate vessels.

To a digester charged with hemlock chips sufficient acid sulfite liquor, analyzing 0.57% combined S02, 6.0% free S02, and 9.1% total solids derived partly from reusing 70% (by volume) of the drained spent acid from a similar preceding digestion, was pumped in to give a total liquidzwood ratio of 5 .3. After steaming to 120 C. in 2.5 hrs., the digester was held at this temperature for 2.0 hrs. and the liquor (containing 14.6% total solids) was drained. The partially cooked (63.1% yield) stock along with waste liquor solids equal in (V0l. liquor-i-vol. water in chips) +Wt. dry chips ,(1./l-:g.).

f'Combingd S02: total SOs-free S02. Total S02 deterrnlned by titration with iodate to starch end-point. Free S02 determined by addingexeess ic-date to the same solution and back titrating with thiosulfate;

weight to the wood solids dissolved during precooking was transferred to a second vessel and N825 and NaOH (25% sulfidity) and water were added to give 0.25 active NazOzoriginal wood at 4.8 total liquidzoriginal wood. The temperature was raised to 160 C. and after 2.0 hrs. at 160 C. the cook was ended. Pulp characteristics were as follows:

Alpha cellulose, 94.1% TAPPI Permanganate No. 5.0

A previous digestion carried out in only one vessel under similar conditions gave a spent alkaline liquor of 19.9% total solids.

Example 4 This example is similar to Example 1 except that a denser wood, southern pine, Was used. Appreciably higher total solids were obtained in both the acid and alkaline stage liquors because of the higher ratio of woody material to liquor used.

Recycled acid sulfite liquor analyzing 22.7% total solids, 0.03% combined S02, and 14.7% free S02 was charged to the digester loaded with southern pine chips in an amount sufficient to give a total liquid:wood ratio of 4.4. The digester was steamed indirectly to C. and held at this temperature for 2.0 hrs. After relieving the digester pressure to essentially atmospheric, the spent acid was drained to an accumulator for use in a subsequent precook. The drained acid was found to contain 25.2% total solids and a small simultaneous basket cook indicated 64.4% precook yield. Suflicient organics-free kraft liquor of 20% sulfidity was pumped to the digester to give 0.13 active alkali at 4.4 total liquid: wood ratio (based on weight of original wood). The digester was steamed indirectly to C. and after 2.0 hrs. at 160 C. was blown. Characteristics of the pulp were as follows:

Alpha cellulose, 96.9% TAPPI Permanganate No. 5.6

in this example the total solids content of the efiluent black liquor amounted to 17.5%.

The invention applies to the digestion of chips from various species of wood, other cellulosic material that may be converted into pulp, and to the use of other bases such as potassium, ammonia, calcium, or magnesium, the latter two being generally less desirable because of a tendency to form insoluble ash. While it is our preferred practice to recycle all of the withdrawn spent acid sulfite liquor in the acid system, some may be withdrawn from the acid circuit (prior to the addition of make-up chemicals) and subsequently disposed of or used as desired.

Primarily, our process applies to the preparation of cellulosic products of high chemical purity, character ized by high-alpha cellulose content. However, by proper adjustment in conditions, it is possible in the practice of our invention, to prepare pulps of highly diverse characteristics. The invention is most advantageously applicable to two-stage digestion processes where it is desirable to obtain the dissolved woody and inorganic waste matter in a spent liquor of highest possible concentration.

Our invention minimizes the amount of equipment required for handling the precooked chips by eliminating the necessity for any washing operation between the acid and alkaline stages.

In contrast to prior suggested methods for preparing acid sulfite preeo'oke'd pulps in which the chemical and heat values of the once-used, low-solids acid stage liquor are not only lost, but also present in aggravating and often costly disposal problem, our process provides for complete disposal of effluents from the pulping operation with complete recovery from one liquor of the pulping chemicalsand all heatvalues derived from the dissolved organic matter. Since no between-stage wash is employed, the single eflluent liquor is obtained at highest possible concentration limited only by the liquidzwood ratios deemed necessary in the acid and alkaline cooks, by the yield of pulp, and by the efiiciency of the brown stock washing system. Thus our process permits operation of a plant without pollution of inlets or other waterways with digester plant effluents at minimum steam consumption.

We claim:

1. In the acid sulfite precooking of wood chips followed by alkaline digestion, the improvement which comprises removing the major proportion of the spent acid sulfite liquor from the precooked chips, fortifying the spent acid sulfite liquor by adding a sufficient amount of chemicals and water to replace the chemicals and water left with the wood chips, using the fortified spent acid sulfite liquor for a similar precooking operation on a fresh batch of chips, subjecting the precooked chips of each batch without washing and containing the residual adhering and imbibed spent acid liquor with its dissolved organic matter to an alkaline digestion, and removing alkaline black liquor from the alkaline digestion containing chemicals and organic matter from both the acid and alkaline stages, said precooking and removal of spent acid sulfite liquor being carried out so that the amount of organic matter in the liquor left with the chips from the precooking is substantially equal to the amount of organic matter removed from the chips in the precooking and the organic matter in the fortified spent acid sulfite liquor reaches a substantially stable value, whereby the alkaline black liquor contains such a concentration of chemicals and organic matter that it can be treated efficiently for the recovery of chemicals and heat.

2. In the method of claim 1, removing free S02 from the spent acid liquor on the chips prior to alkaline digestion.

3. In the method of claim 1, repeating the acid sulfite digestion cyclically and establishing an equilibrium with respect to the organic content of the acid sulfite liquor.

4. In the method of claim 1, removing from the precooked chips as much spent acid sulfite liquor as can be removed by draining.

5. In the method of claim 1, carrying out the alkaline stage with chips containing the residual adhering and imbibed spent acid sulfite liquor and some additional withdrawn spent acid sulfite liquor.

6. In the method of claim 1, producing an alkaline black liquor containing at least 15% of total solids.

7. In the acid sulfite precooking of cellulose-containing material followed by alkaline digestion, the improvement which comprises draining from the precooked material the major proportion of the spent acid sulfite liquor leaving with the material adhering and imbibed spent acid sulfite liquor, fortifying the spent acid sulfite liquor drained from the material by adding a sufiicient amount of base, sulfur dioxide and water to replace these chemicals and water left on the material, using the fortified spent acid sulfite liquor for a similar precooking operation on a fresh batch of material, subjecting the precooked material of each batch without washing and containing the residual adhering and imbibed spent acid sulfite liquor with its dissolved organic matter to treatment for the removal of sulfur dioxide and then to an alkaline digestion, and removing alkaline black liquor from each alkaline digestion containing chemicals and organic matter from both the acid and alkaline stages, said precooking and removal of spent acid sulfite liquor being carried out so that the amount of organic matter in the liquor left with the material from the precooking is substantially equal to the amount of organic matter removed fromthe chips in the precooking and the organic matter in the fortified spent acid sulfite liquor reaches a substantially stable value, whereby the alkaline black liquor contains such a concentration of chemicals and organic matter that it can be treated efiiciently for the recovery of chemicals and heat.

8. In the method of claim 7, removing a small portion of the drained spent acid sulfite liquor from the acid precooking, removing the free sulfur dioxide from the small portion of said liquor and using the said liquor in the alkaline digestion.

References Cited in the file of this patent UNITED STATES PATENTS 1,310,694 Edwards July 22, 1919 1,787,953 Richter Jan. 6, 1931 1,787,954 Richter Jan. 6, 1931 1,864,619 Richter June 28, 1932 2,065,396 Richter Dec. 22, 1936 2,221,066 Kahle Nov. 12, 1940 2,614,923 Tarkkonen Oct. 21, 1952 2,644,748 Cunningham July 7, 1953 FOREIGN PATENTS 583,074 Great Britain Dec. 6, 1946 480,404 Canada Jan. 22, 1952 

1. IN THE ACID SULFITE PRECOOKING OF WOOD CHIPS FOLLOWED BY ALKALINE DIGESTION, THE IMPROVEMENT WHICH COMPRISES REMOVING THE MAJOR PROPORTION OF THE SPENT ACID SULFITE LIQUOR FROM THE PRECOOKED CHIPS, FORTIFYING THE SPENT ACID SULFITE LIQUOR BY ADDING A SUFFICIENT AMOUNT OF CHEMICALS AND WATER TO REPLACE THE CHEMICALS AND WATER LEFT WITH THE WOOD CHIPS, USING THE FORTIFIED SPENT ACID SULFITE LIQUOR FOR A SIMILAR PRECOOKING OPERATION ON A FRESH BATCH OF CHIPS, SUBJECTING THE PRECOOKED CHIPS OF EACH BATCH WITHOUT WASHING AND CONTAINING THE RESIDUAL ADHERING AND IMBIBED SPENT ACID LIQUOR WITH ITS DISSOLVED ORGANIC MATTER TO AN ALKALINE DIGESTION, AND REMOVING ALKALINE BLACK LIQUOR FROM THE ALKALINE DIGESTION CONTAINING CHEMICALS AND ORGANIC MATTER FROM BOTH THE ACID AND ALKALINE STAGES, SAID PRECOOKING AND REMOVAL OF SPENT ACID SULFITE LIQUOR BEING CARRIED OUT SO THAT THE AMOUNT OF ORGANIC MATTER IN THE LIQUOR LEFT WITH THE CHIPS FROM THE PRECOOKING IS SUBSTANTIALLY EQUAL TO THE AMOUNT OF ORGANIC MATTER REMOVED FROM THE CHIPS IN THE PRECOOKING AND THE ORGANIC MATTER IN THE FORTIFIED SPENT ACID SULFITE LIQUOR REACHES A SUBSTANTIALLY STABLE VALUE, WHEREBY THE ALKALINE BLACK LIQUOR CONTAINS SUCCH A CONCENTRATION OF CHEMICALS AND ORGANIC MATTER THAT IS CAN BE TREATED EFFICIENTLY FOR THE RECOVERY OF CHEMICALS AND HEAT. 